TK, the somewhat over-simplified explanation is that to produce very low frequencies at an audible level a sub has to move a lot of air. The obvious way to do this is with a very big driver; a somewhat less obvious way is to use a smaller driver but to have it move farther out and in on each cycle, so that it moves as much or more air than the bigger driver. Designing drivers with long excursions is more difficult and they're more expensive. Also, a more powerful amp is needed to drive them with relatively low distortion at those frequencies(10% or so, compared to the 1% or so inaudible at higher frequencies).
As to whether we can actually hear all this, maybe not, but as Craig pointed out, if it's strong enough we can feel it. Human ears are relatively insensitive things, especially at very low and very high frequencies. The Fletcher-Munson(1930s investigators)curves show that enormously higher levels are required at low frequencies to be audible. Although Fletcher and Munson stopped at 30Hz(apparently they weren't real bass freaks)later investigators extended their work down to about 16Hz, although the required level to be heard at that frequency is so high that only recently have commercially available subs had much chance of supplying it. However, in our lust for ever-lower bass we should be aware of what Alan pointed out to us
here .
